A48.1
BillableLegionnaires' disease
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is A48.1 an HCC code?
Yes. A48.1 maps to Aspiration and Specified Bacterial Pneumonias under the CMS-HCC V28 risk adjustment model (and Aspiration and Specified Bacterial Pneumonias under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for A48.1
For A48.1 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed A48.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
A48.1 is the ICD-10-CM diagnosis code for legionnaires' disease. Legionnaires' disease is a serious lung infection caused by Legionella bacteria, typically acquired from contaminated water sources like air conditioning systems or hot tubs. It causes pneumonia with fever, cough, and breathing difficulties. A48.1 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering other bacterial diseases (a30-a49).
Under the CMS-HCC V28 risk adjustment model, A48.1 maps to Aspiration and Specified Bacterial Pneumonias (HCC 282) with a community, non-dual, aged base RAF weight of 0.711. Under the older V24 model, A48.1 mapped to the same category but with a base RAF weight of 0.339 — V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
Legionnaires' disease is a reportable condition in most jurisdictions; verify reporting requirements in your facility's compliance protocols. Because A48.1 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for A48.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Legionnaires' disease is a reportable condition in most jurisdictions; verify reporting requirements in your facility's compliance protocols
- •Confirm the diagnosis is specifically Legionnaires' disease (A48.1) rather than Pontiac fever (A48.2), which is a milder form of Legionella infection
Clinical Significance
Legionnaires' disease is a severe atypical pneumonia caused by Legionella pneumophila, typically acquired from contaminated water systems (cooling towers, hot water systems, decorative fountains). It carries 5-10% mortality in community cases and up to 25-30% in nosocomial cases. It is clinically significant because standard beta-lactam antibiotics used for typical pneumonia are ineffective — Legionella requires macrolides or fluoroquinolones.
Documentation Requirements
- ✓Legionella urinary antigen test positive (serogroup 1) or respiratory culture/PCR positive for Legionella
- ✓Chest imaging showing pneumonic infiltrates (often rapidly progressive, multilobar)
- ✓Clinical features supporting Legionnaires' disease (high fever, GI symptoms, hyponatremia, elevated LDH, rhabdomyolysis)
- ✓Epidemiologic exposure to potential water source (hospital, hotel, cruise ship, spa)
- ✓Public health notification documentation as this is a nationally notifiable disease